adding a second batt/ and one cap

do your lights actually turn on and off or do they dim? a second battery doesnt do very much for you while your vehicle is running, it is more for reserve capacity while your engine isnt turning your alternator... it sounds like you need neither a cap or a second battery, but a new higher output alternator...if your vehicle cant power what is there, adding batteries and caps will only make that situation worse as your vehicle would then have to recharge the second battery and/or cap, and it is currently struggling to produce enough power to operate what is currently there... yes you could mount a battery inside, but if you were to do that, it would be best to have a sealed battery so there would be no chance of fumes or leakage inside your vehicle...and no, its location is not going to affect SQ in your vehicle, if these questions are still relevant...

on a side note, if you want to run two batteries, you would be best off to buy two brand new identical batteries...if you just add a second battery, unless it is exactly the same (life left, reserve capacity, CCA ratings, etc) you run the risk of prematurely killing both batteries fairly quickly...always buy a matched set if you are going to run two batteries...